1. Field of the Invention
The present invention relates to a semiconductor vertical-cavity, surface-emitting laser, and, more particularly, to a semiconductor vertical-cavity, surface-emitting laser with monolithic and planar structure and having lateral anisotropy in order to control the polarization of the emitted beam of light.
2. Description of the Prior Art
Semiconductor laser diodes, in general, comprise a body of a semiconductor material having adjacent regions of opposite conductivity type forming a p-n junction therebetween. The body is adapted to generate and emit radiation when an appropriate potential is applied across the p-n junction. Vertical-cavity, surface-emitting lasers (VCSELs) emit radiation in a direction perpendicular to the plane of the substrate rather than parallel to the substrate as in the case of conventional edge-emitting diode lasers. In contrast to the elliptical and astigmatic beam quality of conventional edge emitting lasers, VCSELs advantageously emit a circularly symmetric Gaussian beam. Thus, anamorphic correction of the emitted beams of VCSELs is not required. VCSELs moreover, can readily be made into one- and two-dimensional laser arrays as well as be fabricated in extremely small sizes. Accordingly, VCSEL arrays have various applications in the fields of optical memory, laser printing and scanning, optical communications, integrated optoelectronic integrated circuits, optical computing, optical interconnection, etc.
The circular symmetry of the beams emitted from VCSELs arises partly from the fact that they are usually fabricated in a circularly-symmetric structure. It is also well known that VCSELs fabricated in square or even rectangular shaped structures tend to emit circularly-symmetric beams. A consequence of this circular symmetry is the lack of a preferred polarization direction. VCSEL beams emitting in a single transverse mode are linearly polarized. However, the relative direction of the polarization is often different from one VCSEL element to another within an array. For optical systems employing arrays of VCSELs, the variation in polarization direction can greatly degrade system characteristics, such as efficiency and beam uniformity. Because of the high finesse of VCSEL cavities, a slight anisotropy in the optical characteristics can give slight preference to one polarization direction and thus cause the laser to emit a beam polarized in the preferred direction. The semiconductor material forming the VCSELs is crystalline, and the beams show a statistical preference to be polarized in alignment with one or the other major crystal axes, for example in line with the &lt;110&gt; axis or the &lt;010&gt; axis. The ambiguity in polarization direction remains however.
Polarization of VCSEL beams has been controlled by deliberately introducing anisotropies into VCSEL cavities. Anisotropic diffraction losses were introduced by depositing high refractive index material onto opposing sidewalls of an etched VCSEL, thereby stabilizing the polarization as described in an article by Shimazu, et al., entitled "A method of polarization stabilization in surface emitting lasers," published in the JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 30(6A), June, 1991, pgs. L1015-L1017. This method, as described in the article, requires etching through the top mirror, is not effective when the device is larger than about 5 .mu.m across, and this device utilized a hole etched through the entire semiconductor substrate. Another technique, which uses anisotropic strain through a rectangular etch to control polarization has been described in an article by T. Mukaihara, et al., entitled "Stress effect for polarisation control of surface emitting lasers," published in ELECTRONICS LETTERS, vol. 28(6), Mar. 12, 1992, pgs. 555-556. This technique requires etching a rectangular hole through the entire semiconductor substrate. Alternatively, the authors suggested polarization control by externally applying stress in mounting the device. Both of the devices described above used a hole etched through the substrate, resulting in a nonplanar device. This greatly complicates the fabrication and greatly reduces the heat dissipation capability of the devices. The approach of applying an external stress in the mounting results in a non-monolithic device which is subject to nonuniformity and reliability problems. Yet another approach described by Wipiejewski et al., presented at the European Conference on Optical Communication, September 1992, uses an external cavity containing a polarizer to control the polarization of the emitted beam. All these prior art approaches to polarization control suffer serious drawbacks through being nonplanar or requiring external apparatus, and are not considered practical.